The invention relates to household cooking appliances, and more particularly to ceramic slow-cookers.
Modem cookware is often coated with non-stick material to release food quickly and easily, making serving and clean-up less troublesome. Coating metal cookware has been relatively successful where multi-layered coating techniques have been used to improve the bond between the cooking surface and the non-stick coating.
Applying non-stick coatings to ceramic cookware has proven more difficult. The ceramic substrate of conventional ceramic cookware is usually glazed to seal the otherwise porous surfaces, but the glazing does not have the characteristics needed to provide the required non-stick surfaces. The glazing commonly hinders or prevents the application of non-stick coatings to the cooking surfaces. As such, food typically becomes baked onto conventional ceramic cookware, making serving and clean-up difficult. While ceramic cookware with a reliable and long-lasting non-stick coating would be extremely useful, it is currently unavailable on ceramic cookware such as baking dishes, casserole dishes, slow-cookers, etc.
Another problem with applying non-stick coatings to ceramic cookware is the temperature gradients common in the ceramic substrate during cooking or baking. The temperature gradients that exist near the heat source and between the surface of the food and the air provide a harsh environment that tends to degrade and break down non-stick coatings. These temperature gradients add to the above-described difficulty in bonding the non-stick coating to the glazed surfaces and further increase the difficulty of producing ceramic cookware with a reliable non-stick coating.
Different types of ceramic cookware experience different cooking conditions that directly affect the intensity of the temperature gradients. Ceramic cooking appliances such as slow-cookers typically experience more extreme temperature gradients than baking dishes or casserole dishes due in part to the type and location of the heat source and the long cook times associated with slow-cooker applications. Unlike baking or casserole dishes used in the microwave or conventional oven for relatively short cook times, slow-cookers have heating elements in direct or near-direct contact with the ceramic cooking vessel, thereby generating localized and longer-lasting intense heat that creates sharper temperature gradients over the cooking vessel. Also, slow-cookers are often exposed to cool ambient temperatures at the food/air interface. As it is being cooked, the hot food is in contact with a portion of the ceramic cooking vessel while ambient air contacts the vessel just above the surface of the food. This interface experiences another temperature gradient that can be sharper than those associated with baking or casserole dishes used in warmer environments such as ovens or microwaves. As any slow-cooker user knows, it is these areas (the area adjacent the heating element and the interface between the top of the food and the air), that experience the most stuck-on, baked-on and caked-on food remains.
In addition to harsher cooking applications, ceramic slow-cookers are also more difficult to clean than common ceramic cookware. Even assuming that the ceramic cooking vessel of a slow-cooker is removable, it is typically bulky and burdensome to clean. It likely does not fit in a dishwasher and often does not even fit in the sink. If the ceramic cooking vessel is not removable from the slow-cooker housing, clean-up is further complicated as the cooking vessel cannot be submerged due to the electronics inside the housing. Maneuvering the slow-cooker to achieve proper cleaning is difficult and troublesome. This cleaning burden is greatly increased when food is baked on to the ceramic cooking vessel. The excessive scrubbing required to remove the stuck-on food is complicated by the bulky and unmanageable shape and size of the typical slow-cooker.
In light of the problems and limitations of the prior art described above, a need exists for ceramic cookware (and more preferably for a ceramic slow-cooker that has a reliable and long-lasting non-stick coating that simplifies use and cleaning. Each preferred embodiment of the present invention achieves one or more of these results.
The present invention alleviates the problems associated with applying non-stick coatings to ceramic cookware and more particularly to ceramic slow-cookers. The present invention provides a slow-cooker that includes a reliable non-stick coating on the ceramic cooking vessel and provides a method for applying non-stick coating to a ceramic substrate suitable for use with a ceramic slow-cooker. The problems discussed above with respect to using and cleaning slow-cookers call for a non-stick surface that can be applied to the ceramic cooking vessel of a slow-cooker and that can stand up to the harsh cooking environment, thereby simplifying serving and clean-up.
More specifically, the invention provides a slow-cooker comprising a housing, a ceramic cooking vessel received within the housing, the ceramic vessel having interior surfaces defining a cavity and having a non-stick coating covering at least a portion of the interior surfaces and a heating element mounted within the housing for supplying heat to the ceramic vessel.
Preferably, the slow-cooker has a non-stick coating that is multi-layered and includes a first layer preferably having a dry film thickness DFT) of approximately 0.3-0.5 mils. As used herein, the term xe2x80x9cdry film thicknessxe2x80x9d and xe2x80x9cDFTxe2x80x9d refers to the thickness of a film after it is dried from a previously liquid state as is commonly understood in the art. The term xe2x80x9cmilxe2x80x9d refers to a unit of measurement equal to {fraction (1/1000)} of an inch as is commonly understood in the art. More preferably, the non-stick coating includes a first layer having a DFT of approximately 0.4 mils. Also preferably, the non-stick coating includes a second layer having a DFT of approximately 0.3-0.6 mils, and more preferably a second layer having a DFT of approximately 0.4-0.5 mils. Also preferably, the non-stick coating includes a third layer having a DFT of approximately 0.05-0.5 mils, and more preferably a third layer having a DFT of approximately 0.1-0.2 mils.
In most highly preferred embodiments of the present invention, the non-stick coating includes a first layer having a DFT of approximately 0.3-0.5 mils, a second layer having a DFT of approximately 0.3-0.6 mils and a third layer having a DFT of approximately 0.05-0.5 mils. Most preferably, the non-stick coating includes a first layer having a DFT of approximately 0.4 mils, a second layer having a DFT of approximately 0.4-0.5 mils and a third layer having a DFT of approximately 0.1-0.2 mils.
The invention also provides a method for making a non-stick ceramic cooking vessel for a slow-cooker, the method comprising the steps of applying a first layer of a non-stick coating over a ceramic cooking surface of the vessel, drying the first layer, cooling the first layer and applying a second layer of a non-stick coating over at least a portion of the first layer. The method preferably also includes applying a third layer of a non-stick coating over at least a portion of the first and second layers, more preferably before the second layer of non-stick coating is dry. The method is preferably completed by drying the second and third layers and then curing the layers of non-stick coating.
The present invention finds application on both glazed and unglazed ceramic cooking vessels for slow-cookers. Where the present invention is applied to ceramic cooking vessels having a glazing defining a seal over a cooking surface of the ceramic vessel, the method preferably comprises the steps of removing a portion of the glazing while maintaining at least a portion of the seal of the vessel and applying a non-stick coating over the cooking surface. The step of removing a portion of the glazing is preferably achieved by grit-blasting at least a portion of the glazing with aluminum oxide. Additionally, the cooking surface preferably includes a bottom surface and a wall surface wherein at least eighty-five percent of the glazing is preferably removed from at least a portion of the bottom surface and at least fifty percent of the glazing is preferably removed from at least a portion of the wall surface such that.the seal remains substantially intact on the bottom surface and wall surface. More preferably, ninety-five to one hundred percent of the glazing is removed from at least a portion of the bottom surface such that the seal is substantially removed from the bottom surface, and at least 75 percent of the glazing is removed from at least a portion of the wall surface such that the seal remains substantially intact on the wall surface.
More information and a better understanding of the present invention can be achieved by reference to the following drawings and detailed description.